For successful fall seeding, one needs a seed that remains dormant in the fall, survives the winter, and sprouts readily and vigorously in the spring--a difficult combination when every year is climatically different and soil types or conditions throughout the growing regions can vary considerably. Most crop plants being `non-dormant`, germinate readily over a wide range of temperatures in the presence of even small amounts of moisture.
Although most crop plants are `non-dormant`, germination is very species specific; some seeds require light, some darkness, some need a minimum temperature in the 20.degree. C.'s, while others need cool temperatures to germinate. Some require vernalization or exposure to high heat.
Seeds possess germination and dormancy characteristics dependent on their genetic nature. Germination and dormancy vary radically among species. Germination occurs under specific environmental conditions with some variability. Seed of crop plants such as Brassica napus (Argentine canola), B. rapa (Polish canola), B. juncea (brown mustard) and Sinapis alba (yellow mustard) germinate within 4-5 days in the presence of suitable moisture at temperatures as low as 5.degree. C., and within 24 hours as temperatures approach 20.degree. C. The germination tends to be fairly consistent and these seeds are often referred to as being `non-dormant`..sup.3
There are instances where modification of the natural germination/dormancy characteristics of a species will have commercial applications. One potential application is in the area of fall seeding canola in cold winter, short summer growing areas such as the Canadian Prairies and northern United States. Fall seeding canola requires the seed to overwinter in the ground in the dormant state and emerge early in the spring prior to the time a producer can normally work the land. The benefits of fall seeding to the producer are numerous and include: spread of workload, maximum utilization of available soil moisture, and earlier maturity of up to 3 weeks. Yields and grade observed with successful fall seeded canola tend to be higher than with spring seeded crops. The main risk of fall seeding is loss of stand due to premature germination or poor spring vigor. A further drawback, aside from the risk, is the critical timing required for seeding. To be successful, seed must be sown into cold soil just prior to freeze-up, typically affording a window of only a few days. Any unexpected warming after sowing can lead to loss of the stand or poor emergence in the spring. As a consequence, fall seeding canola is currently practiced on a limited scale.
Seed sown in the fall can face numerous conditions which will challenge its subsequent viability in the spring, including fluctuating temperatures and moisture levels, which even if insufficient to induce germination, as exemplified by radicle emergence, may be sufficient to induce early germination-related events such as initiation of breakdown of reserves and structural polysaccharides which can result in poor emergence and vigor in the spring. The fluctuating temperature and moisture levels can also cause tissue damage due to hydration-desiccation after loss of desiccation tolerance, and freezing damage due to hydration followed by a freeze-thaw cycle. The further in advance of freeze-up that the seed is sown, the greater the risk of damage to the seed. To expand the window of opportunity for fall seeding and reduce the risk of loss of stand, all of these factors should be addressed.
Sugars can play an important role in these various processes. For example, respiration and growth of the embryo involve carbohydrate metabolism through the glucose and fructose phosphate pathways and the pentose phosphate pathway..sup.9 The initial softening up of the seed which reduces the physical barrier to radicle emergence involves hydrolysis of the galactomannan polymer cap and other polysaccharides..sup.14,15 Soluble sugars present in a seed can regulate internal osmotic pressures which may affect the amount of moisture required for germination to proceed. In addition, soluble sugars can play a role in stabilizing membranes subjected to desiccation and freezing stresses..sup.10-13
Seeds normally contain a small amount of soluble sugars, typically polysaccharides such as raffinose or monosaccharides such as glucose, although the amount and type is dependant upon the species, and although, addition of sugars or related derivatives to a seed might be expected to affect various properties such as germination, dormancy, seedling vigor, and possibly tolerance to desiccation and freezing, the concept that treatment of canola or related species seed with appropriate sugar solutions would result in that seed being made more suitable for fall sowing in long winter regions is not initially obvious or a priori predictable.
The basic concept that making a `non-dormant` seed less germinable would result in improving its fall seeding was also not a `given`. Originally when abscisic acid and analogs or other known germination inhibitors such as as coumarin were investigated, inhibition of germination in the lab was observed, but improved fall seeding emergence in the field was not. Although increased dormancy may be one factor to consider for fall seeding, the survival of the seed over the winter and the ready and vigorous emergence of the fall-sown seed in the spring are crucial and it would appear may be negatively impacted by some dormancy enhancing treatments.